1. Field of the Invention
This invention relates to a clamping unit of an injection molding machine, which clamping unit comprises a stationary mold carrier, a movable mold carrier, which is movable to and from a clamping position relative to said stationary mold carrier, and hydraulic actuating means for actuating said movable mold carrier, which actuating means comprise a hydraulic locking or power cylinder for applying to said movable mold carrier a locking pressure for locking said movable mold carrier in said clamping position. Said hydraulic locking cylinder contains a piston, which is connected by a piston rod to the movable mold carrier and defines on said cylinder a high-pressure chamber and a low-pressure chamber, which communicates with said high-pressure cylinder through at least one overflow passage. Said actuating means also comprise a hydraulic drive cylinder for moving said movable mold carrier to and from said clamping position. That drive cylinder is symmetrical to an axis of symmetry of the actuating means, and a buffer or compensating cylinder, which has a buffer chamber that communicates with the low-pressure chamber of the locking cylinder.
2. Description of the Prior Art
A known clamping unit of that kind has been disclosed in U.S. Pat. No. 4,846,664, and permits an operation at relatively high speed because the buffer or compensating cylinder causes almost the entire hydraulic oil to remain in the actuating means throughout the operation. A single central hydraulic locking or power cylinder is provided as well as two diametrically opposite buffer or compensating cylinders. But the arrangement of the drive cylinder in a central cavity of the piston rod of the locking or power cylinder and the fact that the locking or power cylinder consists of a steel tube render the supply of the hydraulic fluid to the valve which controls the overflow passage in the piston of the locking cylinder rather difficult.
In another known clamping unit, two cup-like locking cylinders made of cast iron are arranged diametrically opposite to each other on both sides of a central cuplike buffer cylinder made of cast iron and at that end which is nearer to the mold are secured to a mounting plate. A hydraulically separate drive cylinder is disposed in each locking cylinder. The valve member of the valve for controlling the overflow passages in the locking piston is moved to a closed position by pressure applied from the high-pressure chamber and to an open position by pressure applied from a valve chamber. The rate at which the valve can be actuated will be restricted by that design and that mode of operation. Besides, it is very difficult in practice to synchronize the movements of two drive cylinders with sufficient accuracy (EP 0281329, FIG. 8).